This invention relates generally to wireless communication systems, and more particularly to a mechanism for providing variable data rate connections to different users of a Code Division Multiple Access (CDMA) system.
The first generation of personal wireless communication devices, such as cellular telephones, operated on an analog transmission principle using a pair of frequencies per transmission session (sending and receiving) per user. Shortly after analog cellular devices became popular, digital cellular devices were perfected and appeared in the marketplace. Digital cellular devices operate by modulating a digitized voice signal onto a carrier wave. Since the signal is digital, certain techniques can then be used to allow multiple users to access the same frequency spectrum at the same time, thereby effectively increasing system capacity.
One such technique, called Code Division Multiple Access (CDMA), provides a multiple access technique that offers certain benefits over analog cellular transmission techniques. In CDMA, two users may communicate simultaneously using one or more of the same radio frequencies. Each user""s signal is encoded with a unique pseudorandom noise (PN) code at the transmitter in such a way that it can be properly decoded at a receiver with minimal interference to other user signals.
Certain types of interference problems however still exist in CDMA systems. For instance, standard PN codes in and of themselves are not perfectly orthogonal. Thus, using only PN standard codes results in at least some interference between user channels.
In addition, cellular telephones operate at radio frequencies where a fading characteristic occurs when a transmission signal is reflected from many different features of the physical transmission environment. As a result, a transmitted signal may actually be fragmented and arrives at the destination receiver from many directions. These effects can result in destructive summation of the signals, otherwise known as multi-path fading.
Certain modulation techniques can be used to mitigate the adverse effects of interference and multipath fading, while exploiting frequency reuse advantages. One such prior art modulation technique widely implemented in CDMA systems uses mathematically orthogonal codes such as Walsh codes. Using this technique, a digital data signal to be transmitted is first modulated with a pseudorandom noise (PN) code. The resulting signal is then modulated with an orthogonal Walsh code to create a user signal which is orthogonal to other user signals. The Walsh codes are mathematically interrelated such that as long as the coding rate of each channel is the same, different user channels are non-interfering when transmitted on the same frequency.
The benefits of CDMA systems include a decreased requirement for transmission power which results in longer battery life, increased clarity, higher radio frequency re-use, and favorable signal-to-noise ratio results. Since the frequency spectrum can be reused multiple times in such systems, an overall increase in system user capacity also results.
The present invention provides a mechanism to achieve similar results as described above without a fixed rate orthogonal code generator or exact rate orthogonal codes for each channel. Instead, a combination of procedures referred to herein as channel sequencing and bit strobing are used to produce two or more resultant signals that do not interfere with one another when transmitted on the same frequency even at different coding rates. That is, the resultant signals appear to be, and actually are, orthogonal to one another. However, these resultant signals need not be at the same predetermined fixed coding rate, nor is it necessary to use orthogonal Walsh codes.
The invention instead provides for a rolling selection of PN codes such that a resultant signal is xe2x80x9cdouble PNxe2x80x9d modulated. The resultant signal can be transmitted without interference most of the time. However, at times when the value of the rolling channel would produce a double modulated resultant signal that is in an interfering or non-orthogonal state with another users resultant signal, a strobe signal is used to alter the selection of the resultant signal. The strobe signal essentially reverts to selecting the original singly modulated information signal, instead of the double modulated signal.
To provide for the ability to generate non-interfering signals at different coding rates, multiple code modulators are connected in a cascading arrangement. The cascaded arrangement produces output signals such that different output points in the cascade provided with different code rates. All output points in the cascade arrangement generate signals that are orthogonal with each other.
As a result, the coding rates for different users may be individually assigned. By so permitting the customized selection of coding rates for particular users, the affect is that the spreading gain can be also selected for on-a-per-user basis while guaranteeing orthogonality. Thus, for example, users that are communicating over a shorter distance in a cell may use higher data rates with higher coding rates while at the same time users in the same cell at a farther distance out and/or requiring slower data rates may be assigned slower codes. This provides an additional degree of freedom to the system designer in allocating system capacity to users, which previously heretofore has not been available in orthogonally-coded systems.
In one preferred embodiment, the invention provides a system and method for modulating an information signal in a spread spectrum communication system, such as a CDMA communication system. An input information signal to be transmitted can be a digitized voice signal or a data signal generated by a device such as a telephone or modem. A first pseudorandom combiner receives the input information signal and a pseudorandom code sequence. The pseudorandom combiner combines the information signal with the pseudorandom code sequence to produce a first combined signal. The first combined signal alone may be used at certain times as the resultant signal, without further modulation.
A code modulator then operates on the first combined signal to complement the second tier of PN modulation. The code modulator consists of a channel sequence combiner and selector.
The channel sequence combiner receives the first combined signal which includes the information signal in a modulated form. The channel sequence combiner also receives a repetitive maximum length channel sequence. The repetitive maximum length channel sequence is a pseudorandom sequence and not an orthogonal code. The channel sequence combiner combines the first combined signal with the repetitive maximum length channel sequence to produce a second combined signal.
The selector is coupled to receive the first combined signal, the second combined signal and a repetitive strobe signal. The selector selects one of either the first combined signal or the second combined signal to produce a modulated signal based upon a value of the repetitive strobe signal. The repetitive period of the strobe signal is one chip or clock cycle greater in length than the repetitive maximum length channel sequence. As governed by the repetitive strobe signal, there are times when the system of the invention transmits just the first combined signal, instead of the second combined signal which requires the second modulation process.
A cascading arrangement of the code modulators is then provided to achieve the ability to generate non-interfering signals at different coding rates. For example, a three-tiered cascade arrangement provides an output signal that can be selected from as many as one hundred twenty-eight (128) non-interfering signals. Each of the cascaded code modulators operates as the individual code modulator described above, but each is connected in a serial cascading manner so that its respective output signal is received as input to the next following signal generator, and so forth. This provides a hierarchical or cascading arrangement of code modulators in which different levels in the cascade provide different data rates. In addition, all signals generated are non-interfering with one another and none are generated by using orthogonal codes, such as Walsh codes or independent orthogonal code generators.